This invention relates to an improvement in the treatment of whole tailings produced by the hot water extraction process for recovering bitumen from tar sands.
The urgent need for alternative sources of hydrocarbons, especially for use as fuels, is increasingly apparent as reserves of convential crude oil becomes used up. An extensive source of hydrocarbons is in the bituminous sands found in various parts of the world. Particularly useful deposits of bituminous sands are in Western Canada, where they are commonly known as tar sands.
Such tar sands are often found near ground surface; thus they can be mined and transported to an extraction plant for recovery of the heavy oil (bitumen).
The only commercially used process for recovery of bitumen from mined tar sand is the hot water extraction process. In accordance with this process, the tar sand is fed into a rotating conditioning vessel, known as a tumbler, and mixed with hot water, steam, and small amounts of process aid. The most common process aid is sodium hydroxide; its purpose is to assist in causing the bitumen to be released from the other constituents of the tar sand mass. The process is run so that tar sand takes less than 10 minutes to pass through the tumbler. A slurry emerges from the tumbler. This slurry is screened to remove oversize matter, such as rocks or undigested lumps of tar sand, and then is diluted with additional hot water. The screened, diluted slurry is then advanced to quiescent zone known as the primary separation vessel (PSV). Because the components of the slurry are in only loose association (as a result of the conditioning in the tumbler), they are able to separate in the PSV under the influence of gravity. Hence the sand, which has a gravity of about 2.65, sinks to the bottom of the PSV, and may be pumped out in the form of an aqueous mixture. The bitumen has a density of around 1.00, that is, close to that of water. Left to itself, it tends therefore neither to sink nor to float. But a considerable number of air bubbles have been introduced into the slurry in the tumbler and these attach themselves to the bitumen globules. In this way the bitumen, in an aerated state, rises to the top of the PSV and may be collected in the form of a froth. Some bitumen fails to so rise because the size of the globules is too small or because of failure to get aerated. This unaffected bitumen remains in the central region of the PSV and helps make up a portion known as the "middlings". To increase the efficiency of the process, a middlings stream is continuously withdrawn from the PSV and advanced to induced air flotation cells where, by vigorous agitation and the addition of external air, a second yield of bitumen is obtained in the form of a secondary froth. The froths are then combined and the bitumen is separated from the contaminating water and mineral solids. This is done by diluting the combined froths with a hydrocarbon diluent and separating the diluted bitumen using separatory means such as centrifuges. The diluent may then be distilled out of the hydrocarbon product phase to leave pure isolated bitumen that may then be upgraded by processes known in the heavy oil art.
The hot water process is efficient and has the advantage of operating under mild conditions. A disadvantage is the production of a large volume of solids-laden, aqueous tailings. Provision must be made for storing these tailings and, at least as an intermediate step, they must, according to present practice, be impounded within dykes that must be constructed near the mine area. Such tailings ponds bring undesirable environmental effects, and cover tar sand that is thereby rendered unavailable to mining.
One extraction operation, producing 120,000 bbl. synthetic crude per day will, over the 25 year life of the project, create a tailings pond of around 10 square miles in area. If, as other extraction facilities are built, the same area of tar sand is covered, the whole deposit is significantly reduced in size.
In the tailings pond, the solids are supposed to settle to leave a layer of clear water which can be re-cycled to the extraction process. Once enough fresh water has been taken on board, an extraction plant which is self-sufficient in clarified water may be obtained by this recycle process.
In practice, the coarse solids (i.e. the sand grains) do settle rapidly, but the fine solids (i.e. -44 micron fraction) settle only slowly over a period of several years. It would be a great advantage to the tar sand industry if a feasible, rapid water clarification process could be devised.
Workers in the field have turned for guidance to the water clarification art, where large volumes of water are purified for consumption as domestic water. As a result, it has been proposed that coarse solids be first settled out and the fine matter subsequently flocculated by conventional flocculating agents. The flocculated solids could then be removed from the water by centrifuging or filtering. Typical examples of this approach are given in U.S. Pat. Nos. 3,487,003 and 3,502,575, issued to Baillie et al and Hepp et al, respectively.
A new process is the subject of patent application Canadian Ser. No. 311,696, in U.S. Ser. No. 947,996, filed by J. K. Liu et al whose assignees are the same as for the instant invention. The Liu process is based on the discovery that, when flocculant(s) are added to whole tailings (i.e. with the solids unremoved), the coarse particles from nuclei to which the fine particles can adhere. What is produced is a precipitate of aggregates of coarse and fine solids. In this state, the mineral matter can be filtered out without the need for a filter aid. There results a filter cake, small in volume and with little water, and a filtrate sufficiently clear for immediate recycle to extraction. The cake is easy to dispose of because it can be compacted to form a base for reclaimed land. Thus the cake can be disposed of in the mined-out area.
The preferred flocculant in the invention of Liu et al is lime. This is readily available in tar sand regions and does not leave dissolved residues that could be harmful when recycled to extraction. The products of decomposed lime are an insoluble carbonate, and one molecule of water for every reacted molecule of lime. The present invention, that is now summarized, teaches how to optimize the level of flocculant needed.